Alkylamine functionalized metal-organic frameworks for composite gas separations

ABSTRACT

Functionalized metal-organic framework adsorbents with ligands containing basic nitrogen groups such as alkylamines and alkyldiamines appended to the metal centers and method of isolating carbon dioxide from a stream of combined gases and carbon dioxide partial pressures below approximately 1 and 1000 mbar. The adsorption material has an isosteric heat of carbon dioxide adsorption of greater than −60 kJ/mol at zero coverage using a dual-site Langmuir model.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/820,350 filed on Nov. 21, 2017, incorporated herein by reference inits entirety, now U.S. Pat. No. 10,137,430, incorporated herein byreference in its entirety, which is a division of U.S. patentapplication Ser. No. 15/373,426 filed on Dec. 8, 2016, incorporatedherein by reference in its entirety, now U.S. Pat. No. 9,861,953,incorporated herein by reference in its entirety, which is acontinuation of U.S. patent application Ser. No. 14/228,532 filed onMar. 28, 2014, incorporated herein by reference in its entirety, whichis a 35 U.S.C. § 111(a) continuation of PCT international applicationnumber PCT/US2012/060915 filed on Oct. 18, 2012, incorporated herein byreference in its entirety, which claims priority to and the benefit ofU.S. provisional patent application Ser. No. 61/548,676 filed on Oct.18, 2011, incorporated herein by reference in its entirety. Priority isclaimed to each of the foregoing applications.

The above-referenced PCT international application was published as PCTInternational Publication No. WO 2013/059527 on Apr. 25, 2013,incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under DE-SC0001015awarded by the Department of Energy. The Government has certain rightsin the invention.

BACKGROUND OF THE INVENTION 1. Field of Invention

This invention pertains to the use of metal-organic frameworks asadsorbents for the separation of composite gasses, and more particularlyto adsorbents with a high concentration of alkylamine functionalizedsites in a metal organic framework and methods for the separation of avariety of materials based on selective, reversible electron transferreactions. For example, methods are provided for the separation ofindividual gases from as stream of combined gases such as CO₂ from N₂gases or CO₂ from H₂ gases from a stream of combined gases.

2. Background

There is a continuing need for the efficient separation of gas mixturesinto their component parts in many different industrial processesincluding energy production and emission reduction. Many gas separationsare presently performed on large scales in numerous industrialprocesses, often at significant cost.

For example, the production of syngas from the conversion of fossilfuels (natural gas, coal, oil, oil shale, etc) or biomass requires theseparation of CO₂ from H₂ and other useful gasses. In this context, thecoal or other material is converted into syngas (CO and H₂) whichsubsequently undergoes the water-gas shift reaction to generate CO₂ andH₂. The hydrogen is used to generate electricity after it is separatedfrom CO₂, which can then be prevented from release into the atmosphere.This strategy, called pre-combustion CO₂ capture, is advantageous incomparison to other CO₂ capture technologies that require separation ofCO₂ from N₂, O₂, or CH₄ because the differences in size andpolarizability between CO₂ and H₂ can be exploited. Separation of CO₂from CH₄ is also relevant to the purification of natural gas, which canhave impurity levels of up to 92% CO₂ at its source. Carbon dioxideremoval is required for approximately 25% of the natural gas reserves inthe United States. Removal of CO₂, which is most commonly accomplishedusing amines to reduce CO₂ levels to the required 2% maximum, isconducted at pressures between 20 bar and 70 bar. Carbon dioxide removalis required for approximately 25% of the natural gas reserves in theUnited States.

Gas separations are also important in post-combustion of fossil fuelsfor energy production. The combustion of fossil fuels is largelyresponsible for the increase in the global concentration of CO₂ in theEarth's atmosphere, yet fossil fuels will continue to be heavilyutilized for energy production during the 21st century.

The development of more efficient processes for capturing CO₂ from powerplant flue streams is critical for the reduction of greenhouse gasemissions implicated in global warming. Currently, there is significantinterest in the development and implementation of technologies that slowCO₂ emissions and thus forestall the most severe consequences of globalwarming. For limiting future CO₂ emissions from large, stationarysources like coal-fired power plants, carbon capture and sequestration(CCS) has been proposed. The CCS process involves the selective removalof CO₂ from gas mixtures, the compression of pure CO₂ to a supercriticalfluid, transportation to an injection site, and finally permanentsubterranean or submarine storage. For the retrofit of existing powerplants, post-combustion CO₂ capture is a likely configuration. In thisdesign, fuel is burned in air and CO₂ is removed from the effluent. Forcoal-fired power plants, the largest flue gas components by volume areN₂ (70-75%), CO₂ (15-16%), H₂O (5-7%) and 02 (3-4%), with totalpressures near 1 bar and temperatures between 40° C. and 60° C. Forpost-combustion CO₂ capture, maximizing adsorption capacity for CO₂ atlow pressures is highly desirable. Because the partial pressure of CO₂in flue gas emitted from coal fired power stations is typically between0.10 and 0.15 bar, the simplest approximation for the capacity ofmaterials being considered is the quantity of gas adsorbed at theselower pressures, not the capacity at 1 bar.

Aqueous amine solutions are currently the most viable adsorbents forcarbon capture and are presently used for the removal of CO₂ fromindustrial commodities like natural gas. While a variety of advancedamines are available, 30% monoethanolamine (MEA) in water is thebenchmark solvent against which competing technologies are generallycompared. The low solvent cost and proven effectiveness make MEA anattractive adsorbent for many applications.

Conventional CO₂ capture processes involving the chemisorption of CO₂ byalkylamine-containing liquids present several disadvantages, includingthe considerable heat required to regenerate the liquid, solutionboil-off and the necessary use of inhibitors for corrosion control.Therefore, if MEA were to be utilized for carbon capture andsequestration, electricity prices are projected to increase by 86%.

The formation of ammonium carbamate from two MEA molecules and one CO₂molecule endows the scrubber with extremely high selectivity for CO₂,but significant energy is required to regenerate the solution. This highregeneration energy cost has two primary components: first, the strong,chemisorptive bond between the carbon dioxide and the amine must bebroken; second, a large amount of spectator water solvent must be heatedand cooled along with the active amine adsorbent, giving rise to aninefficient system. Because amines are corrosive to plantinfrastructure, solutions are typically limited to no more than 30%(w/w) of the amine, and a significant increase in this concentration isnot deemed feasible. In addition, solvent boil-off occurs duringrepeated regeneration cycles consuming the scrubber and increasingcosts. The diversion of steam from the electricity generation cycle tothe solvent regeneration cycle sharply reduces the net electricityoutput of the plant, drastically increasing electricity costs. It hasbeen demonstrated that plant efficiency is highly dependent on thesolvent regeneration energy that is needed. These limitations representthe most significant obstacles to wider implementation of aminescrubbing technologies for post-combustion carbon capture.

Attempts to address these limitations have focused on the adsorption ofCO₂ in porous solids such as zeolites and amine-modified silicas via theformation of carbamate or bicarbonate species. The viability of thematerials under realistic flue stream conditions requires air and waterstability, corrosion resistance, high thermal stability, and highselectivity for CO₂ over other components in flue gas. Currently,aqueous amines are used industrially to separate CO₂ from gas mixtureswith high CO₂ partial pressures like natural gas, while some solidadsorbents are used to remove CO₂ from mixtures with low CO₂ partialpressures. In addition to the separation of combustion gases, there area number of current industrial processes that utilize liquid or solidadsorbents to remove CO₂ from gas mixtures.

Accordingly, there is a need for an efficient methods and materials forselectively separating constituent gases from a stream of gases that canbe performed at lower temperatures and pressures than existingtechniques. There is also a need for materials and methods that provideselective, reversible electron transfer reactions and associatedfunctions such as catalysis, including oxidation as well as gas storage.The present invention satisfies these needs as well as others and isgenerally an improvement over the art.

SUMMARY OF THE INVENTION

The present invention is directed to metal-organic framework materialsand methods for use in a variety of gas separation and manipulationapplications including the isolation of individual gases from a streamof combined gases, such as carbon dioxide/nitrogen, carbondioxide/hydrogen, carbon dioxide/methane, carbon dioxide/oxygen, carbonmonoxide/nitrogen, carbon monoxide/methane, carbon monoxide/hydrogen,hydrogen sulfide/methane and hydrogen sulfide/nitrogen.

Among the primary benefits of physiorption onto solid materials is thelow regeneration energy compared to that required for aqueous amines.However, this benefit frequently comes at the expense of low capacityand poor selectivity. The present invention provides adsorbents thatbridge the two approaches through the incorporation of sites that bindCO₂ by chemisorption onto solid materials. The new materials mayeliminate the need for aqueous solvents, and may have significantlylower regeneration costs compared with traditional amine scrubbers, yetmaintain their exceptional selectivity and high capacity for CO₂ at lowpressures.

The adsorption materials for gas separations are metal-organicframeworks containing ligands with basic nitrogen groups. Metal-organicframeworks are porous, crystalline solids that are preferablyfunctionalized with the incorporation of alkylamines, which exhibitenhanced basicity over aromatic amines and are capable of stronglyadsorbing acid gases.

The preferred metal-organic frameworks are a group of porous crystallinematerials formed of metal cations or clusters joined by multitopicorganic linkers. By way of example, and not of limitation, the inventionprovides functional materials made from metal-organic frameworkadsorbents with a framework selected from the group: M-BTT (M=Ca, Fe,Mn, Cu, Co, Ni, Cr, Cd); M-BTTri (M=Cr, Mn, Fe, Co, Ni, Cu); M-BTP(M=Co, Ni, Zn); M₃(BTC)₂ (M=Cu, Cr); M₂(dobdc) (M=Mg, Ca, Mn, Cr, Fe,Co, Ni, Cu, Zn); M₂(dobpdc) (M=Mg, Ca, Mn, Cr, Fe, Co, Ni, Cu, Zn), andMIL-100 (M=Fe, Al, Cr, Ti, Sc, V); MIL-101 (M=Fe, Al, Cr, Ti, Sc, V).The metal-organic framework may also include open metal sites.

Ligands of the metal-organic framework may contain other structuralelements used to coordinate the ligand to one or more metals of theframework. These include but are not limited to the following functionalgroups: carboxylate, triazolate, pyrazolate, tetrazolate, pyridines,amines, alkoxide and/or sulfate groups. The preferred alkylamine ligandis N,N′-dimethylethylenediamine (“mmen”) producing (mmen-Mg₂-BTTri) ormmen-Mg₂.(dopbdc) functionalized frameworks.

The removal of dilute concentrations of acid gases by highly selectiveadsorption, as demonstrated by the affinity of (mmen-CuBTTri) for CO₂versus N₂, is used to illustrate the use of the adsorbents inpost-combustion CO₂ capture applications as well as other gas separationapplications.

The basic nitrogen groups may be incorporated into the framework on aligand prior to framework formation, through substitution ormodification of a functional group that was bonded to a ligand prior toframework formation, or by substitution of a ligand after frameworkformation with the ligand with a basic nitrogen group.

Due to their high surface areas and low bulk densities, these materialsdemonstrate remarkable working capacities for sequestering carbondioxide, making them ideal for use in large scale processing plants anda great improvement over current adsorbents. The successfulimplementation of these new adsorbents could reduce the substantialenergy cost of adsorbing CO₂ emissions resulting from the combustion offossil fuels, including coal and natural gas.

Another embodiment is a method of separating a mixture stream comprisingCO₂ and N₂. The method includes contacting the mixture stream includingCO₂ and N₂ with a material comprising a metal-organic framework, and aligand with a basic nitrogen group, wherein the material preferably hasan isosteric heat of CO₂ adsorption of greater than −70 kJ/mol at zerocoverage as determined by the Clausius-Clapeyron relation, obtaining astream richer in CO₂ as compared to the mixture stream, and obtaining astream richer in N₂ as compared to the mixture stream.

According to one aspect of the invention, a process is provided forattaching polyamine ligands to the surface of metal-organic frameworkswith exposed metal cations for use in CO₂ capture.

Another aspect of the invention is to provide a porous adsorbentmaterial with a metal organic framework functionalized withN,N′-dimethylethylenediamine (“mmen”) with frameworks of the familyM-BTTri where M=(Cr, Mn, Fe, Co, Ni or Cu) with H₃[(Cu₄Cl)₃(BTTri)₈(mmen)₁₂] particularly preferred for the separation of gases from amixture of gases at low pressures below approximately 1 bar.

According to another aspect of the invention, a functionalized metalorganic framework is provided that can separate gases at lowtemperatures and pressures.

Yet another aspect of the invention is to provide a material and methodfor pre-combustion separation of carbon dioxide and hydrogen and methanefrom a stream of gases.

A further aspect of the invention is to provide a material and methodfor separation of carbon dioxide from a stream of post-combustion fluegases at low pressures and concentrations.

Another aspect of the invention is to provide a metal-organic frameworkthat is adaptable to many different separation needs.

Further aspects of the invention will be brought out in the followingportions of the specification, wherein the detailed description is forthe purpose of fully disclosing preferred embodiments of the inventionwithout placing limitations thereon.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood by reference to thefollowing drawings which are for illustrative purposes only:

FIG. 1 is a representation of a portion of the structure of the aminefunctionalized metal-organic framework mmen-CuBTTri, with incorporationof the diamine N,N′-dimethylethylenediamine onto open metal sites withinthe pores according to the invention.

FIG. 2A is a graph plotting gravimetric gas sorption isotherms for CO₂(squares) and N₂ (circles) adsorption at 25° C. for mmen-CuBTTri andCuBTTri. The horizontal dashed line in corresponds to 10 wt % CO₂adsorption.

FIG. 2B is a graph plotting volumetric gas sorption isotherms for CO₂(squares) and N₂ (circles) adsorption at 25° C. for mmen-CuBTTri andCuBTTri for comparison to FIG. 2A.

FIG. 3 is a graph of infrared spectra obtained upon exposure ofmmen-CuBTTri to a 5% CO_(2/95)% He gas mixture in a high-pressure DRIFTScell. Under dry conditions, the N—H stretch of mmen is apparent at 3283cm⁻¹ (vertical dashed line) on a fully evacuated sample (28). Dilute CO₂in He was slowly introduced into the cell (30) up to a dynamic pressureof 1.5 bar (32). Upon saturation, the N—H stretch fully disappeared.Following reactivation under vacuum and heating at 60° C. (34), the N—Hstretch reappeared.

FIG. 4 is a graph of CO₂ adsorption isotherms at 298 K (squares), 308 K(circles) and 318 K (triangles) for mmen-CuBTTri.

FIG. 5 is a graph of N₂ adsorption isotherms at 298 K (squares), 308 K(circles) and 318 K (triangles) for mmen-CuBTTri.

FIG. 6 is a graph of Isosteric heats of adsorption for mmen-CuBTTricalculated from the viral method (circles) and the dual-site Langmuirmethod (squares).

FIG. 7 is a graph of % mass change over time with repeated adsorptioncycles. Upon introduction of a 15% CO₂ mixture in N₂ at 25° C., the massof mmen-CuBTTri increased by nearly 7% as measured by thermogravimetricanalysis. Upon saturation, a N₂ purge flow with a temperature swing to60° C. fully regenerated the material, with no apparent capacity lossafter 72 cycles.

FIG. 8A and FIG. 8B depict a synthesis scheme for mmen-Mg₂(dobpdc) where(mmen=N,N′-dimethylethylenediamine) and(dobpdc⁴⁻=4,4′-dioxido-3,3′-biphenyldicarboxylate). From the microwavereaction of MgBr₂.6H₂O and H₄dobpdc, Mg₂(dobpdc) is obtained followingevacuation of the as synthesized solid at high temperatures (middle).Addition of an excess of mmen to the evacuated framework yields theamine-appended CO₂ adsorbent Mg₂(dobpdc)(mmen)_(1.6)(H₂O)_(0.4).

FIG. 9 is a graph of the adsorption of CO₂ in mmen-Mg₂-(dobpdc) at 25°C. (squares), 50° C. (triangles), and 75° C. (circles). Inset: Theisotherms at very low pressures exhibit a step that shifts to higherpressures at higher temperatures. The dashed, vertical line marks thecurrent partial pressure of CO₂ in air (390 ppm).

DETAILED DESCRIPTION OF THE INVENTION

Referring more specifically to the drawings, for illustrative purposesseveral embodiments of the metal-organic framework adsorbents of thepresent invention are depicted generally in FIG. 1 through FIG. 9 andthe associated methods for using and producing the alkylaminefunctionalized frameworks. It will be appreciated that the methods mayvary as to the specific steps and sequence and the metal-organicframework architecture may vary as to structural details, withoutdeparting from the basic concepts as disclosed herein. The method stepsare merely exemplary of the order that these steps may occur. The stepsmay occur in any order that is desired, such that it still performs thegoals of the claimed invention.

Metal-organic frameworks are a class of porous, crystalline adsorbentsthat enables greater functionality with reduced adsorbent mass andvolume compared to traditional solid adsorbents. These metal-organicframeworks are preferred because of the presence of coordinativelyunsaturated metal centers (open metal sites) along the pore surfaces.These coordinate metal cations are known to behave as Lewis acids thatstrongly polarize gas adsorbents and are further amenable topost-synthetic functionalization. In these frameworks with wellseparated open metal sites, one amine of a diamine ligand molecule canbind to a metal cation as a Lewis base while the second amine remainsavailable as a chemically reactive adsorption site. The metals in theframework can be individual metal atoms bridged by a set of ligands ormetal clusters (a collection of metal atoms that as a group interactwith a set of ligands.

The preferred metal-organic frameworks are a group of porous crystallinematerials formed of metal cations or clusters joined by multitopicorganic linkers. These are frequently frameworks that are described ashaving “open metal sites” (also called coordinatively unsaturated metalcenters).

By way of example, and not of limitation, the invention providesfunctional materials made from metal-organic framework adsorbentsselected from the group: M-(1,3,5-benzenetristriazolate) where (M=Cr,Mn, Fe, Co, Ni or Cu). Other basic framework examples that can befunctionalized with alkylamines and used for low pressure applicationsinclude: M-BTT (M=Ca, Fe, Mn, Cu, Co, Ni, Cr, Cd)(BTT=1,3,5-benzenetristetrazolate); M-BTP where (M=Co, Ni, Zn) and(BTP=1,3,5-benzenetrispyrazolate); M₃(BTC)₂ where (M=Cu, Cr) and(BTC=1,3,5-benzenetriscarboxylate); M₂(dobdc) where (M=Mg, Ca, Mn, Cr,Fe, Co, Ni, Cu, Zn) and (dobdc=2,5-dioxido-1,4-benzenedicarboxylate);M₂(dobpdc) where (M=Mg, Ca, Mn, Cr, Fe, Co, Ni, Cu, Zn) and(dobpdc=4,4′-dioxido-3,3′-biphenyldicarboxylate); MIL-100 where (M=Fe,Al, Cr, Ti, Sc, V) and (Ligand=BTC=1,3,5-benzenetriscarboxylate); andMIL-101 where (M=Fe, Al, Cr, Ti, Sc, V) and(Ligand=BDC=1,4-benzenedicarboxylate).

The ligands of the metal-organic frameworks preferably contain basicnitrogen groups. These basic nitrogen ligands may include, for example,alkyl amines or imines, but not aromatic amines (i.e. anilinederivatives).

Some or all ligands of the metal-organic framework include functionalgroups that are not coordinated to metal cations and are available toform reversible weak chemical bonds with CO₂. Preferably, the reactivechemical atom contains a lone pair of electrons including nitrogen,oxygen, sulfur, and phosphorous. More preferably, it is a basic amine.More preferably, the lone pair or pairs of the reactive atom are not inresonance with an aromatic ring. Most preferably, the functional groupis a primary, secondary, or tertiary alkylamine (an aliphatic amine).

Ligands of the metal-organic framework may contain other structuralelements used to coordinate the ligand to one or more metals of theframework. These include but are not limited to the following functionalgroups: carboxylate, triazolate, pyrazolate, tetrazolate, pyridines,amines, alkoxide and/or sulfate groups. The preferred alkylamine ligandis N,N′-dimethylethylenediamine (“mmen”) producing (mmen-Mg₂-BTTri) ormmen-Mg₂.(dopbdc) functionalized frameworks.

Some or all ligands of the metal organic framework may contain one ormore aromatic rings that contain carbon and may contain other atomsincluding boron, nitrogen, and oxygen. This is most preferably five andsix membered rings. These rings may provide structural rigidity to thematerial and/or provide spatial separation of other functional groupscontained within ligands as to provide porosity to the adsorbent.

The structure of the basic ligands may include three distinctcomponents: 1) A backbone that provides structural rigidity to thematerial. This may be, for example, an aromatic group such as a phenylgroup. 2) At least one functional group that binds the ligands to themetal such as nitrogen or oxygen atoms. Specific examples include acarboxylate group, a triazolate group (as in this case for Cu-BTTri),pyrazolates, tetrazolates, pyridines, and sulfates. 3) A functionalgroup that contains a nitrogen atom that is not integral to thestructural rigidity of the material and is not bound to a metal that isavailable to interact with gases.

The functional group that contains a nitrogen atom and interacts withthe CO₂ molecule is preferably a basic organic group. Preferredfunctional groups include primary amines, secondary amines, tertiaryamines, primary imines, and secondary imines, and similar compounds.Although these groups all contain nitrogen, in alternative embodiments,these groups could include other atoms as well, especially atoms havingan available lone pair of electrons.

These basic nitrogen functional groups can be incorporated into themetal-organic framework in one of three preferable ways: 1) attached asa functional group on a ligand prior to framework formation; 2) as asubstitution or modification of a functional group that was bonded to aligand prior to framework formation; and 3) as a substitution of aligand after framework formation for a new ligand that contains thedesired functional group.

If the amines are attached to the ligand prior to framework formation asin method 1) described above, for incorporating a basic nitrogen groupinto the framework, the nitrogen is preferably not directly bonded to anaromatic carbon atom. This is because the nitrogen would be bonded to analkylcarbon (a methylene), giving rise to an alkylamine groups, which ispreferred. Accordingly, it is preferred that at least one atom of anytype separates the amine that interacts with CO₂ from the aromaticbackbone.

In method 2), the basic nitrogen groups are incorporated into theframework through a substitution or modification of a functional groupthat was bonded to a ligand prior to framework formation and the ligandsare not exchanged. Rather, functional groups within the ligands may beexchanged for the desired functionality. This could potentially includemodification of C—H bonds by aromatic substitution reactions,nucleophilic substitution reactions (including replacement of alkylhalide groups for alkyl amine groups), condensation reactions (includingconversions of aldehydes to imines), and reductions (including imines toalkyl amines, nitriles to alkylamines, and amides to alkyl amines).Other potential reactions could also be used that modify ligands toinclude alkylamines or imines after framework synthesis.

Finally, with method 3) for incorporating a basic nitrogen group intothe framework, the metal ligands are exchanged. The new ligandpreferably has at least two functional groups: 1) A functional groupused to bind CO₂ and 2) a functional group used to bind to the metal.The second functional group that binds the metal can also be an amine.It is possible to use other functional groups such as oxygen containinggroups like alcohols, ethers or alkoxides, carbon groups like carbenesor unsaturated bonds like alkenes or alkynes, or sulfur atoms.

Preferable characteristic for the end that binds the metal include thefollowing: 1) strongly bonded to the metal so the functional groups arenot removed upon framework activation by vacuum; 2) capable of beinggrafted at nearly all metal sites within the pores for nearly completefunctionalization. The ligand itself may contain one or more amines thatbind CO₂. For example, the ligand could have 3 carboxylate groups forbinding metals and 1, 2, or 3 (or more) alkylamine groups on eachligand. Examples may include Tris(2-aminoethyl)amine (primary andtertiary amines) or Tris[2(methylamino)ethyl]amine (secondary andtertiary amines), which would be capable of binding a metal-site withone amine and adsorbing CO₂ with 3 other amines. These examples arebranched.

Other alternatives are linear like tetraethylenepentamine (2 primaryamines and 3 tertiary amines) or Diethylenetriamine (2 primary aminesand 1 secondary amine). In the case of mmen-CuBTTri, described herein,it is believed that each amine binds one CO₂. However, it is alsopossible for two or more amines to bind a single CO₂, especially inligands with multiple amines.

Amines do not necessarily simply polarize CO₂; rather, they strongly andselectively bind it through chemisorptive interactions. Amines tetheredto solid surfaces within porous materials also have considerableadvantages over aqueous alkanolamines. It has also been found that theincorporation of alkylamine groups at higher loadings can furtherpolarize the overall surface area of a metal-organic framework, therebyincreasing the capacity for CO₂ capture. Other functional groups aresimilarly capable of polarizing framework surfaces, but many are notcapable of undergoing the chemisorptive type process. Higher orderamines, in particular secondary amines, have more favorable adsorptioncharacteristics in solutions as well as on solid adsorbents. It has beenfound that the incorporation of N,N′-dimethylethylenediamine (mmen) athigh loadings within CuBTTri affords a material with exceptional CO₂capture characteristics.

Generally, a method for separating constituent gases from stream ofmixed gases containing at least a first gas and a second gas is providedwith the use of an adsorbent of a metal-organic framework adsorbent ofthe group M-BTT (M=Ca, Fe, Mn, Cu, Co, Ni, Cr, Cd); M-BTTri (M=Cr, Mn,Fe, Co, Ni, Cu); M-BTP (M=Co, Ni, Zn); M₃(BTC)₂ (M=Cu, Cr); M₂(dobdc)(M=Mg, Ca, Mn, Cr, Fe, Co, Ni, Cu, Zn); M₂(dobpdc) (M=Mg, Ca, Mn, Cr,Fe, Co, Ni, Cu, Zn); MIL-100 (M=Fe, Al, Cr, Ti, Sc, V) or MIL-101 (M=Fe,Al, Cr, Ti, Sc, V) that have been functionalized with ligands containingbasic nitrogen groups including but not limited to alkyldiamine ligands.The stream of mixed gases is directed across a bed of adsorbent and themolecules of the first gas are adsorbed onto the metal-organic frameworkso that the resulting stream is richer in the second gas as compared tothe mixture stream that is collected. The adsorbed first gas is releasedfrom the metal-organic framework to obtain a stream richer in the firstchemical as compared to the mixture stream that is also collected. Theadsorbed chemical is typically released by a change in temperature orpressure. A purge gas may also be used to move the released gas throughthe bed for collection.

It will be seen that the selection of the metal cations and organicframework structure can be tailored by the type of gases to be separatedand the temperature and pressure conditions of the separation.M₂(dobpdc) and CuBTTri functionalized with alkylamines are frameworksthat are particularly suited for carbon dioxide/nitrogen gas separationsat CO₂ partial pressures between 1 and 1000 mbar.

Two frameworks, mmen-CuBTTri shown in FIG. 1 and mmen-Mg₂(dobpdc) shownin FIG. 8B, are used to illustrate the M-BTTri and M₂(dobpdc) frameworkfamilies and the methods of use for gas separations.

Turning now to FIG. 1, an embodiment of a portion of a metal-organicframework crystal structure of a mmen-CuBTTri, a water stable,triazolate-bridged framework of the invention is schematically shown.The incorporation of the N,N′-dimethylethylenediamine (mmen) ligand intothe (CuBTTri) framework, was shown to drastically enhance CO₂adsorption. The attachment of the mmen alkylamine at the metal centersof the framework is shown with arrows. Because the diamines are shorterthan the distance between two adjacent metal sites one amine from eachmolecule is bound to a single metal site, while the other amine is freeto interact with guest gas molecules upon framework activation.

High porosity is maintained despite stoichiometric attachment of mmen tothe open metal sites of the framework, resulting in a BET surface areaof 870 m²/g. At 25° C. under a 0.15 bar CO₂/0.75 bar N₂ mixture,mmen-CuBTTri adsorbs 2.38 mmol CO₂/g (9.5 wt %) with a selectivity of327, as determined using Ideal Adsorbed Solution Theory (IAST). The highcapacity and selectivity are consequences of the exceptionally largeisosteric heat of CO₂ adsorption, calculated to be −96 kJ/mol at zerocoverage. Infrared spectra support chemisorptions between amines and CO₂as one of the primary mechanisms of uptake. Despite the large initialheat of adsorption, the CO₂ uptake was fully reversible and theframework could be easily regenerated at 60° C., enabling a cycling timeof just 27 minutes with no loss of capacity over the course of 72adsorption/desorption cycles.

Overall, the performance characteristics of CuBTTri indicate it to be anexceptional new adsorbent for CO₂ capture, comparing favorably with bothamine-grafted silicas and aqueous amine methods.

The large capacity, high selectivity, and fast kinetics of thesematerials for adsorbing CO₂ from dry gas mixtures with N₂ and O₂ makethe functionalized metal-organic frameworks attractive new adsorbentsfor applications in which zeolites and liquid adsorbents are currentlyused, including the removal of CO₂ from flue gases at low pressuresbelow approximately 1 bar.

The invention may be better understood with reference to theaccompanying examples, which are intended for purposes of illustrationonly and should not be construed as in any sense limiting the scope ofthe present invention as defined in the claims appended hereto.

Example 1

In order to demonstrate the functionality of metal-organic frameworksfeaturing coordinatively-unsaturated metal centers for separating gases,a mmen-CuBTTri framework as shown in FIG. 1 was constructed and tested.

A sample of CuBTTri (100.0 mg, 32.4/μmol) was suspended in 10 mL ofanhydrous hexane under nitrogen, and 75.4 μL (61.2 μg, 701 μmol, 1.8equivalents per unsaturated CU^(II) site) of N,N′dimethylethylenediamine (mmen) was added via micropipette with stirring.The compound immediately turned blue and the suspension was heated atreflux for 18 hours under nitrogen. The solid was collected byfiltration and washed with successive aliquots of hexane (5×10 mL) toremove unreacted diamine. The solid was then dried under reducedpressure to remove hexane. Anal. Calcd. for C₁₄₄H₁₉₅Cl₃Cu₁₂N₉₆(Mw=4139.6 g/mol): C, 41.78, H, 4.75, N, 32.48. Found: C, 42.23, H,4.47, N, 32.05.

The grafted material, mmen-CuBTTri, was then activated by heating at 50°C. for 24 hours under a dynamic vacuum prior to gas adsorption. Nitrogenadsorption isotherms collected at 77 K indicate a BET surface area of870 m²/g, while powder x-ray diffraction data show the structure of theCuBTTri framework to be intact. Overall, the characterization data aremost consistent with a chemical formula of H₃[(CU₄Cl)₃(BTTri)₈(mmen)₁₂],with approximately one mmen molecule for each available metal site.Thus, mmen-CuBTTri is thought to possess a high concentration ofsurface-appended mmen molecules, where one of the amine groups is boundto a Cu²⁺ center, while the other dangles within the pore, as depictedin FIG. 1.

Using the same procedure, another framework:H₃[(Cu₄Cl)₃(BTTri)₈(men)₆].2H₂O (men-CuBTTri) was produced and evaluatedfor comparison. 1.8 equivalents of N-methylethylenediamine (men) wasadded to activated CuBTTri in hexane, giving rise a blue material. Anal.Calcd. for C₁₁₄H₁₁₅C₁₃CU₁₂N₈₄O₂ (Mw=3562.6 g/mol): C, 38.43, H, 3.25, N,33.02. Found: C, 38.55, H, 3.23, N, 32.51.

Example 2

In order to demonstrate the functionality of metal-organic frameworksfeaturing coordinatively-unsaturated metal centers for separating gases,a mmen-CuBTTri framework as shown in FIG. 1 was constructed and tested.

The mmen-CuBTTri and men-CuBTTri adsorbents were initially tested in thecontext of separating a mixture stream including CO₂ and N₂ to obtain astream richer in N₂ as compared to the mixture stream, with theadsorption of CO₂ in the frameworks at low pressures.

Gas adsorption isotherms for pressures in the range 0-1.1 bar weremeasured by a volumetric method using a Micromeritics ASAP2020instrument. A sample was transferred in an N₂ filled glovebag to apre-weighed analysis tube, which was capped with a transeal andevacuated by heating (50° C. for mmen-CuBTTri, 100° C. for men-CuBTTri)under dynamic vacuum for 24 hours. The evacuated analysis tubecontaining the degassed sample was then carefully transferred to anelectronic balance and weighed again to determine the mass of sample(108.5 mg for mmen-, 69.2 mg for men-CuBTTri). The tube was thentransferred back to the analysis port of the gas adsorption instrument.For all isotherms, warm and cold free space correction measurements wereperformed using ultra-high purity Helium gas (UHP grade 5.0, 99.999%purity); N₂ isotherms at 77 K were measured in liquid nitrogen usingUHP-grade gas sources. CO₂ and N₂ isotherms at 298, 308 and 318 K weremeasured using a Julabo isothermal bath with UHP-grade gases. Oil-freevacuum pumps and oil-free pressure regulators were used for allmeasurements to prevent contamination of the samples during theevacuation process or of the feed gases during the isothermmeasurements.

The incorporation of mmen in the CuBTTri framework resulted in amaterial with excellent CO₂ adsorption characteristics. As shown in FIG.2A and FIG. 2B, mmen-CuBTTri displays significantly enhanced CO₂adsorption at all pressures between 0 and 1.1 bar relative to theunappended framework.

Thermogravimetric analyses were carried out at a ramp rate of 1° C./minunder a nitrogen flow with a TA Instruments TGA Q5000 V3.1 Build 246.CO₂ cycling experiments were performed using 15% CO₂/N₂ (PraxairCertified standard NI-CD15C-K) and N₂ (Praxair, 99.99%). A flow rate of25 mL/min was employed for both gases. Prior to cycling, the sample wasactivated by heating at 60° C. for 1 hour, followed by cooling to 25° C.under an N₂ atmosphere. Sample mass was normalized to be 0% at 25° C.under an N₂ atmosphere. Masses were uncorrected for buoyancy effects,which were small compared to mass changes realized from gas adsorption.

The crystallite volumetric capacities of CuBTTri and mmen-CuBTTri weredetermined from unit cell densities. Only slight changes to the unitcell length were apparent from the PXRD diffraction patterns uponaddition of mmen to the open-metal sites of CuBTTri. For both samples, aunit cell length of a=18.647 A and a unit cell volume of V=6483.8 A3were used for density calculations. The molecular weight of mmen-CuBTTriwas calculated to have 1 mmen for each open metal site, while CuBTTriwas calculated to have no guest solvent molecules present. Gas sorptiondata was converted from mmol/g to mmol/cm³ with the density of CuBTTri:p=0.789 g/cm³ D and the density of mmen-CuBTTri: p=1.059 g/cm³

Quantifying the extent of the improvement in CO₂ adsorption betweenCuBTTri and mmen-CuBTTri is not trivial, since the degree of improvementdepends significantly on the units to which the gas uptake isnormalized. FIG. 2A plots the gravimetric gas adsorption isotherm, whileFIG. 2B plots the crystallite volumetric gas adsorption isotherm for thetwo materials.

FIG. 2A is a graph plotting gravimetric gas sorption isotherms for CO₂(squares) 12 and N₂ (circles) 16 adsorption at 25° C. for mmen-CuBTTri.Gravimetric gas sorption isotherms for CO₂ (squares) 14 and N₂ (circles)18 for adsorption at 25° C. for CuBTTri alone is also plotted forcomparison. The horizontal dashed line in corresponds to 10 wt % CO₂adsorption.

FIG. 2B is a graph plotting volumetric gas sorption isotherms for CO₂(squares) 20 and N₂ (circles) 24 adsorption at 25° C. for mmen-CuBTTri.Volumetric gas sorption isotherms for CO₂ (squares) 22 and N₂ (circles)26 adsorption at 25° C. for CuBTTri alone is also plotted for comparisonfor comparison to FIG. 3A.

The volumetric capacity for an actual adsorber unit is dependent uponhow crystallites pack together and the fraction of void space within theoccupied volume. Yet, gravimetric capacity alone does not provide acomplete measure of the performance of a material being proposed forstationary applications, such as post-combustion CO₂ capture. Here,infrastructure costs are linked more directly to the volume theadsorbent would occupy than to its mass. Because incorporation of mmeninto CuBTTri increases the framework density by 34% with no significantchange in volume, this system is a good candidate for comparisonsbetween gravimetric and volumetric capacities.

It is important to note, however, that no single-crystal diffractiondata are available for either CuBTTri or mmen-CuBTTri. Framework volumesare based upon powder pattern unit cell optimizations, and frameworkcompositions are based upon elemental and thermogravimetric analyses.

At 25° C. and 1 bar, mmen-CuBTTri adsorbs 4.2 mmol/g of CO₂ (15.4 wt %),representing a 15% improvement in gravimetric capacity compared to theunmodified CuBTTri framework. However, CO₂ comprises at most 15% of coalfired power station flue gas and the effluent is released into theenvironment at total pressures near 1 bar. Thus, the more importantcriterion for CO₂ capacity is that of the framework at a pressure near0.15 bar. At 25° C. and 0.15 bar of CO₂, mmen-CuBTTri adsorbs 2.38mmol/g (9.5 wt %). Note that 2.90 mmol/g would correspond to theadsorption of one CO₂ molecule per mmen in the functionalized framework.Under the same conditions, the unmodified framework only adsorbs 0.69mmol/g (2.9 wt %). Thus, on a gravimetric basis, mmen-CuBTTri adsorbsnearly 3.5 times as much CO₂ at the relevant pressures. Volumetrically,however, mmen-CuBTTri adsorbs about 4.7 times more CO₂ at 0.15 bar thanCuBTTri. The difference between gravimetric and volumetric densities isa direct consequence of the increased mass of the appended frameworkover the unappended material.

At 25° C., mmen-CuBTTri adsorbs less N₂ than CuBTTri at all pressuresbetween 0.0 and 1.1 bar. This is due to the reduction in specificsurface area upon incorporation of mmen, with the BET surface area of870 m²/g for mmen-CuBTTri being roughly half of the 1770 m²/g observedfor CuBTTri. The additional polarizing sites in mmen-CuBTTri enhance N₂adsorption less than the decreased surface area diminishes N₂adsorption. The opposite trend was observed for CO₂ adsorption. Enhancedadsorption of only one gas is a defining characteristic ofchemisorption. In contrast, frameworks replete with open metal cationsites can be expected to polarize all gases more effectively, includingN₂, accounting for the substantially greater N₂ adsorption in Mg₂(dobdc)relative to mmen-CuBTTri.

The selectivity (S) for adsorption of CO₂ over N₂ in mmen-CuBTTri wasestimated from the single-component isotherm data. For CO₂ capture, thisvalue typically reports the ratio of the adsorbed amount of CO₂ at 0.15bar to the adsorbed amount of N₂ at 0.75 bar; the value is normalizedfor the pressures chosen. The values are derived from an approximateflue gas composition of 15% CO₂, 75% N₂, and 10% other gases, at a totalpressure of 1 bar. Pure-component isotherm selectivities, whichfrequently are calculated from the excess adsorption data directlymeasured by gas adsorption, can be misleading. The adsorptionselectivity, S_(IAST), was therefore modeled by applying the IdealAdsorbed Solution Theory (IAST) to the calculated absolute adsorptionisotherms. The accuracy of the IAST procedure has been established forthe adsorption of a wide variety of gas mixtures in different zeolites,as well as CO₂ capture in metal-organic frameworks. For mmen-CuBTTri,SIAST values were calculated to be 327, 200, and 123 at 25° C., 35° C.,and 45° C., respectively. The selectivity that was observed at 25° C. isone of the highest values reported for a metal-organic framework.

Example 3

To further characterize the mmen-CuBTTri frameworks, the isosteric heatof adsorption and working capacity of the materials were analyzed.Utilizing a dual-site Langmuir adsorption model, isosteric heats ofadsorption were calculated for CO₂ in mmen-CuBTTri and compared to thoseobtained from data for bare CuBTTri, which were fit using a single-siteLangmuir model to give a value of −24 kJ/mol. The N₂ adsorption isothermfor mmen-CuBTTri was also fit to a single-site Langmuir model, resultingin a calculated isosteric heat of adsorption of −15 kJ/mol.

The isosteric heat of CO₂ adsorption in mmen-CuBTTri approaches −96kJ/mol at zero coverage, corresponding to the largest value yet reportedfor CO₂ adsorption in a metal-organic framework. For comparison tommen-CuBTTri, the heat of adsorption for en-CuBTTri was recalculatedwith the same dual-site Langmuir model. Because this model incorporatesabsolute adsorption, direct comparisons between the two different modelsare not possible. From the dual-site Langmuir model, the isosteric heatof CO₂ adsorption in en-CuBTTri was calculated to be −78 kJ/mol at zerocoverage, nearly 20 kJ/mol lower in magnitude than the heat calculatedfor mmen-CuBTTri. Preferably, the isosteric heat of CO₂ adsorption ofthe material is greater than −70 kJ/mol at zero coverage as determinedby the Clausius-Clapeyron relation.

Accordingly, mmen-CuBTTri has a significantly larger number of freeamines available to bind guest CO₂ molecules. Because isosteric heatscorrespond to the average of all adsorption sites potentially populatedat a specific coverage level, at zero coverage there is a higherprobability of the CO₂ molecule adsorbing onto an amine in mmen-CuBTTricompared with en-CuBTTri.

Despite the large binding enthalpy between alkylamines and CO₂, thesubstantial decrease in the −Q_(st) data with loading indicates thatmany weaker adsorption sites are also being sampled by the CO₂ moleculesunder the conditions probed. While amine sterics may play a role in theimproved adsorption properties of mmen-CuBTTri, it is not possible tomake comparisons between isosteric heats of adsorption as a function ofamine sterics because of the differences in amine loading levels.

The significantly greater working capacities of strongly bindingadsorbents may lead to materials that are less expensive to operate thanthose that have smaller working capacities. Solid adsorbents with largeisosteric heats of adsorption have considerable advantages includinghigh selectivity and high capacity for CO₂ at low partial pressures;however, they are often believed to be the most difficult to regenerate.Regeneration, however, is very dependent on the method best suited to agiven material. For carbon capture from flue gas streams, vacuum andtemperature swing adsorption methodologies are the ones most frequentlyenvisioned. Vacuum swing adsorption can best be approximated by thedifference in capacities between the adsorption and desorptionpressures. For mmen-CuBTTri, a 7 wt % working capacity between 0.15 and0.02 bar at 25° C. was calculated. For temperature swing methods,adsorbents with high heats of adsorption may prove to be bettercandidates than materials with moderate heats of adsorption. This isbecause the capacities of adsorbents with high heats of adsorption aremore dependent on temperature than materials with smaller heats ofadsorption.

The cyclability of the mmen-CuBTTri material as a CO₂ adsorbent wasevaluated using a combined temperature swing and nitrogen purgeapproach. Utilizing a thermogravimetric analyzer, a mixture of 15% CO₂in N₂ was introduced into the furnace for 5 minutes at 25° C. It wasobserved that the sample mass increased by nearly 7% upon introductionof the mixed gas, due to strong adsorption of CO₂, even in the dilutemixture. The adsorbent was then regenerated by changing the flow to apure N₂ stream followed by rapid ramping of the furnace at 5° C./min to60° C. The sample was held for 2 minutes at 60° C., and then cooled at5° C./min to 25° C. The temperature was allowed to stabilize for 2minutes, followed by the reintroduction of the 15% CO₂ in N₂ mixture.The 27 minute cycling procedure was repeated 72 times, with no apparentchange in capacity as shown in FIG. 7. The kinetics of the adsorptionare sufficiently quick that little additional CO₂ is adsorbed after thefirst few minutes, and even shorter cycle times could be utilized withonly small reductions in capacity. Similarly, complete desorption isrealized prior to the end of each 2 minute isotherm. Importantly, thecycling capacity of mmen-CuBTTri under dry conditions is comparable toor even greater than the working capacity of a 30% MEA solution, whichis frequently reported as 5.5 wt %.

Example 4

Heats of adsorption approaching −100 kJ/mol and highly specificinteractions are indicative of chemisorptive processes. In the absenceof water, it is believed that the electrophilic CO₂ molecule isaccepting electron density from the lone pair of the free amine of mmen,forming a zwitterionic carbamate. Previous spectroscopic studies of CO₂binding in amine containing metal-organic frameworks have investigatedonly less basic aromatic amines. For example, it was recently shown inNH₂-MIL-53(AI) that the amine was not directly interacting with the CO₂,but rather, other physisorptive processes account for the favorableadsorption characteristics of the material.

Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS)measurements were performed on mmen-CuBTTri using a high-pressure (0-3bar) gas cell to confirm and characterize the proposed chemisorptiveprocess. Infrared spectra were collected on a Perkin Elmer Spectrum 400FTIR spectrometer equipped with an attenuated total reflectanceaccessory (ATR). For Diffuse Reflectance Infrared Fourier TransformSpectroscopy (DRIFTS) spectra, the FTIR spectrometer was equipped aBarrick Praying Mantis Diffuse Reflectance accessory and a high-pressuregas cell with temperature control. 5% CO₂ in Be (Praxair certifiedstandard BE CD5C-K) and a vacuum were attached to the high-pressurecell. Gas was slowly introduced into an evacuated cell containingmmen-CuBTTri prepared in a N₂ filled glovebag. Maximum pressuredelivered to the cell was 1.5 bar above atmospheric pressure. Followingadsorption, the sample was regenerated at 60° C. under dynamic vacuumfor two hours.

FIG. 3 plots the infrared absorbance of mmen-CuBTTri under variouspressures of CO₂ in a 5% CO_(2/95)% He gas mixture in the high-pressureDRIFTS cell. Under dry conditions, the N—H stretch of mmen is apparentat 3283 cm⁻¹ (vertical dashed line) on a fully evacuated sample is shownat plot 28. The reported frequency for the N—H stretch in free mmen is3279 cm⁻¹. Dilute CO₂ in He was slowly introduced into the cell at plot30 up to a dynamic pressure of 1.5 bar at plot 32. Upon saturation, atotal disappearance of the N—H band at 3283 cm⁻¹ is clearly observedwith the introduction of 5% CO₂ in He in the cell at increasingpressures. Upon regeneration of the solid under vacuum and heating at60° C. at plot 34, the N—H stretching band returned. A sharp band at1386 cm⁻¹ also appears in the spectra as CO₂ is introduced; diffusebands near 1669, 1487, and 1057 cm⁻¹ are also apparent in addition tochanges in the fingerprint region.

Water was strictly excluded from the material for the DRIFTSmeasurements to eliminate the possibility of carbonate formation.Because the amines are tethered to the framework and well separated, wedo not believe that it is possible for two amines to be concertedlyinteracting with a single CO₂ molecule. Similar measurements have beenperformed on other porous materials, most notably amine-graftedmesoporous silicas. In these materials, however, two amines can oftenact concertedly and the experiments frequently incorporated water vapor,making direct comparisons difficult. It has been previously reported,however, in dry amino acid-based ionic liquids that zwitterionicCO₂-species exhibit sharp infrared band near 1660 cm⁻¹. From the largecalculated heat of adsorption and observed band changes in the infraredregion upon CO₂ addition, it is believed that the primary mechanism ofCO₂ adsorption at low pressures is the chemisorption of CO₂ gas ontommen molecules resulting in the formation of zwitterioinc carbamates orcarbamic acid.

Example 5

The steric effect upon diamine incorporation was also evaluated. Theframework CuBTTri was additionally modified with N methylethylenediamine(men), an asymmetric diamine with one primary and one secondary amine.The adsorption isotherms of mmen-CuBTTri for CO₂ at 298 K (squares), 308K (circles) and 318 K (triangles) is shown in FIG. 4 as a baseline forcomparison. FIG. 5 shows N₂ adsorption isotherms at 298 K (squares), 308K (circles) and 318 K (triangles) temperatures for mmen-CuBTTri.

Relative to the performance of mmen-CuBTTri, men-CuBTTri performs moresimilarly to en-CuBTTri, for which only a small enhancement in CO₂uptake was observed at very low pressures. A small improvement inperformance was, however, realized through the use of men over en.

The significantly greater adsorption of CO₂ in mmen-CuBTTri isattributable to the larger number of amines that are accessible to guestCO₂ molecules. There are two primary factors affecting the incorporationof primary amines into CuBTTri. First, the best fits to the elementalanalysis data indicate at least two times as many diamine molecules wereincorporated into mmen-CuBTTri versus the en- and men-analogues. Areasonable explanation for the higher incorporation of mmen into theframework is the formation of a weaker coordinate bond between thesecondary amine on mmen and a framework Cu²⁺ ion compared with therelatively stronger coordinate bond formed between a primary amine and aCu²⁺ ion. The significantly greater reversibility of mmen binding withCu²⁺ imparts it with a faster diffusion rate through the pores such thatmmen molecules that bonded with copper sites near pore openings werelabile in the hot hexane solution employed for grafting. These diamineswere capable of migrating deeper into pores to achieve high surfacecoverage. In the case of primary amines, the irreversibility of thecoordinate bond at the synthesis conditions surveyed severely limitedthe diffusion of en and men into the pores. Once appended, additionaldiamines could only diffuse through constricted pores to reach interiormetal sites. Note that in men-CuBTTri, the primary amine end of men iscoordinated to the metal centers while the sterically hindered secondaryamine is available to interact with guest molecules in the pores.Increased reversibility of en and men binding was sought through the useof higher boiling solvents and lower concentrations; however, graftingtemperatures that significantly exceeded 100° C. resulted in thedecomposition of the framework.

Second, based upon the calculated heats of adsorption for en- andmen-CuBTTri, the number of amines that were strongly adsorbing CO₂ wassignificantly less than the number of amines that were appended in eachframework. This is because not all of the amines in the en- andmen-frameworks bonded to open metal sites. Grafted amines severelyreduced the pore diameters, slowing diffusion rates as previouslydiscussed. Some of these pores then became blocked by the excess amines.Amines and other adsorption sites beyond these blockages wereinaccessible to guest gases. This was confirmed by the low surface areascalculated for en- and men-CuBTTri. By comparison, the surface area formmen-CuBTTri was significantly higher despite the greater number oftotal amines calculated to be within the framework.

Example 6

Isosteric heats of adsorption are commonly calculated after fittingadsorption equilibrium data to the Virial equation. FIG. 6 is a graph ofisosteric heats of adsorption for mmen-CuBTTri calculated from thevirial method (circles) and the dual-site Langmuir method (squares).FIG. 6 overlays the isosteric heats of adsorption calculated formmen-CuBTTri using both the dual-site Langmuir and the Virial methods.At zero coverage, the Virial method gives a significantly lowermagnitude for the heat of adsorption: −66 kJ/mol.

However, inflections in the isotherms tor mmen-CuBTTri are notaccurately modeled with the Virial method. It is therefore believed thatthe values of the dual-site model are significantly more accurate at lowCO₂ loadings. In contrast, at intermediate loadings there is goodagreement between the dual-site Langmuir and Virial fits. Whilemmen-CuBTTri exhibits a large heat of adsorption at zero loading, heatsof adsorption at intermediate loadings are also important for CO₂capture. At a loading of 2.4 mmol/g, the approximate capacity ofmmen-CuBTTri for CO₂ at 0.15 bar, the isosteric heat of adsorption wascalculated to be about −45 kJ/mol by both the dual-site Langmuir andVirial models. Hence, in CO₂ capture applications, the average enthalpyof adsorption for CO₂ would be significantly less than the −96 kJ/molvalue calculated for very low coverage levels. This has importantimplications for adsorbent regeneration.

The measured experimental data on pure component isotherms for CO₂ andN₂, in terms of excess loadings, were first converted to absoluteloading using the Peng-Robinson equation of state for estimation of thefluid densities. The pore volume of mmen used for this purpose was 0.363cm³/g, based on the N₂ adsorption data at 77 K. The pore volume obtainedwas 51% that of bare CuBTTri.

The absolute component loadings were fitted with either a single-siteLangmuir model or a dual-site Langmuir model. For N₂/mmen there are nodiscernible isotherm inflections and therefore the single-site Langmuirmodel was used for isotherm fitting. The single-site Langmuir model isalso adequate for fitting the isotherm data for CO₂ in “bare” CuBTTri.In order to fit the experimental data for adsorption of CO₂ inmmen-CuBTTri, the dual-site Langmuir model was employed.

Due to their high surface areas and low bulk densities, these materialsdemonstrate remarkable working capacities for sequestering carbondioxide, making them ideal for use in large scale processing plants anda great improvement over current adsorbents. The successfulimplementation of these new adsorbents could both reduce the substantialenergy cost of hydrogen purification and reduce or eliminate CO₂emissions in the generation of electricity from coal or syngas.

Example 7

To further demonstrate the broad functionality of the metal-organicframework family M₂(dobpdc) (M=Mg, Mn, Fe, Co, Cu, and Zn), theframework was used for the separation of CO₂ from mixed nitrogen gases.The M₂(dobpdc) structure type features approximately 18 Å-wide channelsand exhibits exceptional CO₂ adsorption properties uponfunctionalization with mmen. Several different metal organic frameworkswere prepared for comparison testing.

Referring to FIG. 8A and FIG. 8B, one synthesis scheme for themmen-M₂-(dobpdc) functionalized framework is illustrated. As shownschematically in FIG. 8A, the initial framework is produced fromH₄dobpdc (4,4′-Dihydroxy-(1,1′-biphenyl)-3,3′-dicarboxylic Acid).H₄dobpdc was produced by adding 4,4″-dihydroxybiphenyl (1.16 g, 6.24mmol), KHCO₃ (2.00 g, 20.0 mmol), dry ice (4 g), and1,2,4-trichlorobenzene (3 mL) to a PTFE insert within a steel aciddigestion bomb (23 mL) and heated at 255° C. for 17 hours. After coolingto room temperature, the mixture was collected via vacuum filtration andwashed with diethyl ether. The solid was suspended in 300 mL ofdistilled water, and filtered again. To the filtrate, neat HCl wasslowly added until a pH between 1 and 2 was reached. The resulting crudeproduct was collected via filtration. Recrystallization using 50 mL ofacetone and 50 mL of water per gram of crude material afforded 0.68 g(40%) of pure product as a white powder.

Reaction of H₄dobpdc with ZnBr₂.2H₂O or MgBr₂.6H₂O in 1:1 DEF: EtOHproduces Zn₂(dobpdc)(DEF)₂.DEF.H₂O (DEF-1) andMg₂(dobpdc)(DEF)₂.DEF_(1.5).H₂O (DEF-2), respectively as seen in FIG.8A. DEF-1 was produced by mixing H₄dobpdc (4.0 mg, 0.015 mmol),ZnBr2.2H₂O (8.9 mg, 0.034 mmol), and 0.5 mL of mixed solvent (1:1DEF:EtOH; DEF=N,N′-diethylformamide) a 2-mL Pyrex tube. The tube wassealed and placed in a preheated oven at 100° C. After 72 hours,needle-shaped, colorless crystals had formed. The crystals were isolatedby filtration and washed with hot DEF to afford 3.7 mg (35%) of product.

Similarly, DEF-2 was produced by loading H₄dobpdc (24 mg, 0.088 mmol),MgBr₂.6H₂O (60 mg, 0.21 mmol), and 3 mL of solvent (1:1 DEF:EtOH) into a10 ml Pyrex cell and sealed with a PTFE cap. The mixture was irradiatedin a microwave reactor (CEM Discover) for 30 minutes at 120° C. After 30minutes, the solution was cooled and the resulting solid was collectedvia filtration and washed with hot DEF. The solid was dried under vacuumto yield 57.5 mg (95%) of product as a white powder.

Upon exposure of DEF-2 to atmospheric air, the white powder turns blue.Amine functionalization, however, appears to enhance frameworkstability, because no similar degradation was observed for mmen-2 uponexposure to air for one week.

In the crystal structure of DEF-1, four different dobpdc⁴⁻ ligands andone DEF molecule are bonded to each Zn²⁺ ion in a distorted octahedralgeometry. There are three unique 0 donor types from the dobpdc⁴⁻ ligand:bridging (μ₂) aryloxide 0 atoms (01), bridging (μ₂) carboxylate O atoms(O₂), and non-bridging carboxylate O atoms (O₃). The equatorial plane ofeach Zn²⁺ is composed of two trans-disposed O1 ligands from differentlinkers, one O3 donor atom, and one O2 donor atom. An O2 donor atomoccupies one axial coordination site, while the other axial site isoccupied by an O donor atom from DEF, the reaction solvent. Thiscoordination mode results in the formation of helical chains of Zn²⁺atoms running along the c axis of the crystal. The resulting frameworkconsists of a honeycomb lattice of hexagonal, one-dimensional channelsapproximately 18 A in width. Bound DEF molecules occupy the Zn²⁺coordination sites along the corners of hexagonal channel walls. PowderX-ray diffraction (PXRD) data indicate DEF-2 to be isostructural withDEF-1.

Heating DEF-1 or DEF-2 at 420° C. for 65 minutes in vacuo yielded thefully activated adsorbent Mg₂(dobpdc) or Zn₂(dobpdc) respectively.Heating DEF-2 at 420° C. for 65 min under dynamic vacuum, removed theDEF molecules bound to the metal atoms, completely activating thematerial and generating open Mg²⁺ coordination sites. Such extremethermal treatment was necessary because soaking in methanol at 100° C.for 20 hours did not lead to exchange of the bound DEF molecules. Theporosity of activated DEF-2 was confirmed via N₂ adsorption at 77 K,resulting in a BET surface area of 3270 m²/g. Note that, in line withthe expanded structure, this is significantly greater than the BETsurface area of 1495 m²/g reported for Mg₂(dobdc).

The synthesis and structure of mmen-Mg₂(dobpdc) or mmen-2 is depictedschematically at the right of FIG. 8A and FIG. 8B. An activated sampleof DEF-1 of DEF-2 was suspended in hexanes and an excess of mmen wasadded. Specifically, a 77 mg, 0.24 mmol sample of activated DEF-2 wasimmersed in anhydrous hexane, and 20 equivalents ofN,N′-dimethylethylenediamine (mmen, 0.53 mL, 4.8 mmol) was added. Thesuspension was stirred for one day, filtered, and rinsed copiously withhexanes. The solid was then evacuated of residual solvents at 100° C.for 24 h to afford 87 mg (77%) of product as a gray-white powder.

As shown by powder X-ray diffraction, framework crystallinity was notsignificantly affected by activation or subsequent aminefunctionalization. A much reduced BET surface area of 70 m²/g wascalculated for mmen-2, while DFT pore size distributions indicated areduction in average pore size.

Example 8

CO₂ adsorption isotherms were taken of the mmen-2 functionalizedframeworks and the bare Mg₂(dobpdc) framework for comparison. Isostericheats of adsorption for Mg₂(dobpdc) were calculated to be −44 kJ/mol atlow coverage. This value is 42 kJ/mol higher in magnitude than the −42kJ/mol previously reported for the analogous Mg₂(dobdc) framework.

The adsorption capacity of Mg₂(dobpdc) at 25° C. is 4.85 mmol/g (13.8 wt%) and 6.42 mmol/g (20.0 wt %) at 0.15 and 1 bar, respectively. Thecapacity of Mg₂(dobpdc) for CO₂ at 0.15 bar exceeds the capacity of mostmetal-organic frameworks.

The alkylamine-functionalized metal-organic framework mmen-2 displayedan extremely high affinity for CO₂ at extraordinarily low pressures. TheCO₂ adsorption isotherms obtained at 25, 50, and 75° C. are presented inFIG. 9. At 25° C. and 0.39 mbar, near the current partial pressure ofCO₂ in Earth's atmosphere, the compound adsorbed 2.0 mmol/g (8.1 wt %),which is 15 times the capacity of Mg₂(dobpdc). At the much higherpressure of 5 mbar, the median partial pressure of CO₂ within theInternational Space Station, the framework adsorbed 2.6 mmol/g (10.3 wt%). For comparison, zeolite 5 A, which is currently used aboard thestation to adsorb CO₂, adsorbs 0.85 mmol/g (3.6 wt %, crystallographicvolumetric capacity 1.3 mmol/cm³) at 5 mbar.

At 25° C., the CO₂ adsorption in mmen-2 reaches 3.14 mmol/g (12.1 wt %)at 0.15 bar and 3.86 mmol/g (14.5 wt %) at 1 bar. Remarkably, its CO₂uptake at 1 bar and 25° C. exceeds the amount of N₂ adsorbed at 77 K.Thus, the low surface area measured at 77 K does not appear toaccurately reflect the surface area accessible to CO₂. On a per massbasis, the amine-functionalized framework adsorbed less CO₂ thanMg₂(dobpdc) at pressures higher than ca. 0.1 bar. While the decreasedsurface area of mmen-2 may limit its capacity at super-atmosphericpressures, the large density difference between the two frameworks isprimarily responsible for the lower gravimetric capacity of mmen-2.Crystallographic densities of 0.58 and 0.86 g/cm³ were calculated forMg₂(dobpdc) and mmen-2, respectively. For adsorbents of the samestructure and widely different densities, volumetric capacities betterrepresent CO₂ adsorption performance. At 0.15 bar, Mg₂(dobpdc) andmmen-2 adsorb 2.1 and 2.7 mmol/cm³, respectively, while at 1 bar, thecapacities of both adsorbents are 3.3 mmol/cm³.

For stationary applications like CCS, the greater volumetric capacity ofmmen-2 makes it the superior adsorbent. Based upon the calculated numberof dangling amine groups in mmen-2, a loading of 3.4 mmol/g wouldcorrespond to one CO₂ per amine, yet uptake of only ca. 2.2 mmol/g wasobserved. Here, pore blockages, hydrogen bonded amines, or cooperativebinding mechanisms between two amines and one CO₂ may be limiting theaccessible stoichiometry of mmen-2. Thus, significant additionalcapacity improvements might be realized in the material if conditionscan be identified for appending one mmen per magnesium and binding oneCO₂ molecule per dangling amine.

Isosteric heat of adsorption calculations were hindered by the presenceof a prominent step in the isotherms at low pressures and convex to thepressure axis. Generally, continuous mathematical functions are used tomodel experimental isotherms, which then become the input parameters forthe Clausius-Clapeyron relation. Since mathematical modeling of the CO₂isotherms of mmen-2 with continuous equations over the entire pressurerange were unavailable, each isotherm was modeled with twoLangmuir-Freundlich equations. Data sets corresponding to the adsorptionregions before and after the steps were compiled and then modeledindividually. Isosteric heats of adsorption for mmen-2 were calculatedfrom the 25, 50, and 75° C. isotherm models. At low loadings, heatssignificantly lower than those expected for chemical adsorption of CO₂onto an amine were calculated. However, calculated heats quicklyapproached and maintained a value of −71 kJ/mol, which likelycorresponds to the chemical adsorption of CO₂ onto the free amine ofmmen. Here, a carbamate with a weak C—N bond is probably formed throughinteraction the lone pair of the free amine of mmen and theelectrophilic carbon of CO₂.

In situ diffuse reflectance infrared Fourier transform spectroscopy(DRIFTS) was employed to probe the chemical nature of adsorbed CO₂. At25° C., 1 bar of 5% CO₂ in Helium was introduced into an air-tight gascell containing a sample of activated mmen-2. The difference spectrumbetween mmen-2 under a 1 bar atmosphere and the activated frameworkunder vacuum was obtained. A prominent loss peak was observed at 3316cm⁻¹ and assigned to the NH stretch of free mmen-2, and is indicative ofchemical adsorption of CO₂ onto amines. Gain bands observed at 1702 cm⁻¹and 1531 cm⁻¹ likely correspond to carbamate C═O and CHN groupstretches, respectively.

Recent work on alkylamine grafted silica surfaces have suggested thatchemical adsorption of CO₂ onto alkylamines is not possible withoutneighboring amines or surface hydroxyl groups to stabilize the resultingcarbamates; ammonium carbamates or surface bonded carbamates are formed,respectively. The formation of surface bonded carbamates in mmen-2 isunlikely due to a lack of surface hydroxyl groups, and the broad NHstretches expected for ammonium cations are not definitively resolvablefrom the DRIFTS difference spectrum. Furthermore, the slow reversibilityof mmen-2 at room temperature appears to preclude the formation ofammonium carbamates, which have been reported to desorb CO₂ from primaryamines readily at room temperature.

The step observed in each isotherm marks the pressure at which CO₂adsorption becomes dominated by chemisorption. Interestingly, the stepmoves to significantly higher pressures as the adsorption temperatureincreases. The location of the step is modeled well by a simpleClausius-Clapeyron relation, which predicts how isotherms move as afunction of temperature. The existence of the step, however, isunexpected in a strongly adsorbing material with large pores, and canbest be explained by the surprising conclusion that weaker adsorptionsites are energetically favored over amine adsorption sites at lowcoverage. This suggests that adsorption of CO₂ onto mmen is disfavoredat low adsorptive concentrations (the density of gas phase CO₂ in thepores) because of the large positive entropy associated withreorganization of the amines, as required to form a chemical bond withCO₂.

Example 9

To evaluate the performance of mmen-2 as a regenerable adsorbent for CO₂capture from air, thermogravimetric analysis (TGA) was utilized tomonitor sample mass under dynamic environments. The changes in samplemass (normalized to the mass of the framework under N₂ at 25° C.) whilesimulated air containing 390 ppm CO₂ was flowed over the sample wereobtained. Despite the very low concentration of CO₂, a 4.6% mc (%mc=percent mass change; 1.05 mmol/g; 4.4 wt %) was realized after 60min.

Thermogravimetric analyses were carried out at ramp rates between 5 and10° C./min under a nitrogen flow with a TA Instruments TGA Q5000 (Ver.3.1 Build 246) or a Scinco TGA N-1000. Carbon dioxide cyclingexperiments were performed using a 15% CO₂ in N₂ (Praxair NI-CD15C-K),390 ppm CO₂ in air (Praxair AI-CD-390C-K; 390 ppm CO₂, 21% O₂, balanceN₂), CO₂ (Praxair 99.998%) and N₂ (Praxair, 99.9%). A flow rate of 25mL/minute was employed for all gases. Prior to cycling, the sample wasactivated by heating at 150° C. for 1 hour. The sample mass wasnormalized to be 0% at the adsorption temperature (25° C. for 390 ppmCO₂ and 40° C. for 15% CO₂) under flowing N₂ and sample mass wasnormalized to be 0% at 150° C. under flowing 100% CO₂.

The adsorbent was then regenerated under flowing N₂ at 150° C. for 30minutes and the cycle repeated ten times with no apparent loss ofcapacity. The equilibrium capacity (2.0 mmol/g, 1.72 mmol/cm³) of mmen-2is similar to the capacities of the very best amine-grafted silica andalumina adsorbents reported to date. However, the kinetics of adsorptionappear to be significantly faster in mmen-2 than for amines depositedvia evaporation or polymerization methodologies. For example, while thepseudo-equilibrium capacity of an outstanding poly(ethyleneimine)impregnated silica gel was reported to be 2.4 mmol/g, it took nearly 200minutes for the silica based adsorbent to realize 4.6% mc, the capacityof mmen-2 for CO₂ after only 60 minutes. The easily accessed amineswithin mmen-2 appear to enhance adsorption rates greatly, enabling rapidadsorption-desorption cycles to be utilized.

Example 10

The capabilities of mmen-2 as an adsorbent for removing CO₂ from theflue gas of coal-fired power stations were also investigated. Thedynamic cycling behavior of mmen-2 under the relevant, dry conditions:15% CO₂ in N₂ flowing over the solid at 40° C. was also investigated.

After adsorbing CO₂ for 15 minutes, the sample was heated at 120° C. for15 minutes under N₂. A capacity of 11.1% mc (2.52 mmol/g, 9.9 wt %)relative to the sample mass of mmen-2 under N₂ at 40° C. was realized.After 50 cycles, only a 0.2% mc capacity loss was observed. Longeradsorption and desorption times did not significantly improve thecycling capacity of the material, nor did higher desorptiontemperatures. It was noted that the apparent capacity of mmen-2 greatlyexceeds the ca. 2 wt % working capacity of aqueous monoethanolamine(MEA) scrubbers, which would likely swing between the same adsorptionand desorption temperatures.

If captured CO₂ is to be sequestered, high purity CO₂ is essential. Todesorb the ca. 98% pure CO₂ adsorbed onto mmen-2, a N₂ or air purgecannot be utilized to strip the adsorbent bed. Hence, to approximate theworking capacity of mmen-2 using a temperature swing without a N₂ purge,a pure CO₂ purge was utilized instead. A 7.8% mc (1.8 mmol/g, 7.2 wt %)was realized when 15% CO₂ in N₂ at 40° C. was desorbed with 100% CO₂ at150° C.

From the discussion above it will be appreciated that the invention canbe embodied in various ways, including the following:

1. An adsorption material, comprising: a porous metal-organic framework;and a plurality of ligands within the pores of the metal-organicframework, each ligand having at least one basic nitrogen group; whereinthe basic nitrogen groups are configured to selectively adsorb CO₂ froma stream of mixed gases at pressures below approximately 3 bar and CO₂partial pressures between approximately 1 and 1000 mbar.

2. The adsorption material of any previous embodiment, wherein themetal-organic framework is a framework selected from the group offrameworks consisting essentially of M-BTT where (M=Ca, Fe, Mn, Cu, Co,Ni, Cr, Cd) and (BTT=1,3,5-benzenetristetrazolate) and M-BTTri where(M=Cr, Mn, Fe, Co, Ni, Cu) and (BTTri=1,3,5-benzenetristriazolate).

3. The adsorption material of any previous embodiment, wherein themetal-organic framework is a framework selected from the group offrameworks consisting essentially of M-BTP where (M=Co, Ni, Zn)(BTP=1,3,5-benzenetrispyrazolate) and M₃(BTC)₂ where (M=Cu, Cr) and(BTC=1,3,5-benzenetriscarboxylate).

4. The adsorption material of any previous embodiment, wherein themetal-organic framework is a framework selected from the group offrameworks consisting essentially of MIL-100 where (M=Fe, Al, Cr, Ti,Sc, V) and Ligand=BTC=1,3,5-benzenetriscarboxylate) and MIL-101 (M=Fe,Al, Cr, Ti, Sc, V) and (Ligand=BDC=1,4-benzenedicarboxylate.

5. The adsorption material of any previous embodiment, wherein themetal-organic framework comprises M₂(dobdc) (M=Mg, Ca, Mn, Cr, Fe, Co,Ni, Cu, Zn) (dobdc=2,5-dioxido-1,4-benzenedicarboxylate).

6. The adsorption material of any previous embodiment, wherein the basicnitrogen group is incorporated into the framework on a ligand prior toframework formation.

7. The adsorption material of any previous embodiment, wherein the basicnitrogen group is incorporated into the framework through substitutionor modification of a functional group that was bonded to a ligand priorto framework formation.

8. The adsorption material of any previous embodiment, wherein the basicnitrogen group is incorporated into the framework by substitution of aligand after framework formation with the ligand with a basic nitrogengroup.

9. The adsorption material of any previous embodiment, wherein theligand comprises a first functional group reactive to metal atoms in themetal-organic framework and a second functional group reactive withcarbon dioxide.

10. The adsorption material of any previous embodiment, wherein thefirst functional group of the ligand comprises a phenyl group.

11. The adsorption material of any previous embodiment, wherein thefirst functional group of the ligand comprises a carboxylate group, atriazolate group, a pyrazolate, a tetrazolates, a pyridine, or asulfate.

12. The adsorption material of any previous embodiment, wherein theligand comprises a primary alkylamine, a secondary alkylamine, atertiary alkylamine, a primary imine, or a secondary imine.

13. The adsorption material of any previous embodiment, wherein themetal-organic framework comprises open metal sites and ligand occupiedmetal sites.

14. The adsorption material of any previous embodiment, wherein theadsorption material has an isosteric heat of CO₂ adsorption of greaterthan −60 kJ/mol at zero coverage using a dual-site Langmuir model.

15. A method of separating a mixture stream comprising CO₂ and N₂, themethod comprising: contacting the mixture stream comprising CO₂ and N₂with a material comprising a metal-organic framework, and a ligand witha basic nitrogen group; wherein the material has an isosteric heat ofCO₂ adsorption of greater than −60 kJ/mol at zero coverage using adual-site Langmuir model; obtaining a stream richer in CO₂ as comparedto the mixture stream; and obtaining a stream richer in N₂ as comparedto the mixture stream.

16. The method as recited in any previous embodiment, wherein themetal-organic framework is a framework selected from the group offrameworks consisting essentially of M-BTT where (M=Ca, Fe, Mn, Cu, Co,Ni, Cr, Cd) and (BTT=1,3,5-benzenetristetrazolate); M-BTTri where (M=Cr,Mn, Fe, Co, Ni, Cu) and (BTTri=1,3,5-benzenetristriazolate); M-BTP where(M=Co, Ni, Zn) (BTP=1,3,5-benzenetrispyrazolate); M₃(BTC)₂ where (M=Cu,Cr) and (BTC=1,3,5-benzenetriscarboxylate); MIL-100 where (M=Fe, Al, Cr,Ti, Sc, V) and Ligand=BTC=1,3,5-benzenetriscarboxylate); MIL-101 where(M=Fe, Al, Cr, Ti, Sc, V) and (Ligand=BDC=1,4-benzenedicarboxylate, andM₂(dobdc) (M=Mg, Ca, Mn, Cr, Fe, Co, Ni, Cu, Zn) where(dobdc=2,5-dioxido-1,4-benzenedicarboxylate).

17. The method as recited in any previous embodiment, wherein the ligandis selected from the group of ligands consisting essentially of acarboxylate group, a triazolate group, a pyrazolate, a tetrazolates, apyridine, or a sulfate.

18. The method as recited in any previous embodiment, wherein the ligandcomprises a primary alkylamine, a secondary alkylamine, a tertiaryalkylamine, a primary imine, or a secondary imine.

19. A method of separating a mixture stream comprising CO₂ and othercombustion gases, the method comprising: contacting the mixture streamcomprising CO₂ and N₂ with a material comprising a metal-organicframework and a plurality of ligands that have at least one basicnitrogen group; obtaining a stream richer in CO₂ as compared to themixture stream; and obtaining a stream richer in N₂ as compared to themixture stream.

20. The method as recited in any previous embodiment, wherein themetal-organic framework and plurality of ligands comprises mmen-CuBTTri.

21. An adsorption material comprising: a metal-organic framework; and aligand with a basic nitrogen group, wherein the adsorption material hasan isosteric heat of CO₂ adsorption of greater than −80 kJ/mol at zerocoverage using a dual-site Langmuir model.

22. The adsorption material of any previous embodiment, wherein thebasic nitrogen group is incorporated into the framework on a ligandprior to framework formation.

23. The adsorption material of any previous embodiment, wherein thebasic nitrogen group is incorporated into the framework throughsubstitution or modification of a functional group that was bonded to aligand prior to framework formation.

24. The adsorption material of any previous embodiment, wherein thebasic nitrogen group is incorporated into the framework by substitutionof a ligand after framework formation with the ligand with a basicnitrogen group.

25. The adsorption material of any previous embodiment, wherein theligand comprises a carboxylate group, a triazolate group, a pyrazolate,a tetrazolates, a pyridine, or a sulfate.

26. The adsorption material of any previous embodiment, wherein theligand comprises a primary amine, a secondary amine, a tertiary amine, aprimary imine, or a secondary imine.

27. The adsorption material of any previous embodiment, wherein themetal-organic framework comprises open metal sites.

28. The adsorption material of claim 1, wherein the adsorption materialhas an isosteric heat of CO₂ adsorption of greater than −95 kJ/mol atzero coverage using a dual-site Langmuir model.

29. A method of separating a mixture stream comprising CO₂ and N₂comprising: contacting the mixture stream comprising CO₂ and N₂ with amaterial comprising a metal-organic framework, and a ligand with a basicnitrogen group, wherein the material has an isosteric heat of CO₂adsorption of greater than −80 kJ/mol at zero coverage using a dual-siteLangmuir model; obtaining a stream richer in CO₂ as compared to themixture stream; and obtaining a stream richer in N₂ as compared to themixture stream.

Although the description above contains many details, these should notbe construed as limiting the scope of the invention but as merelyproviding illustrations of some of the presently preferred embodimentsof this invention. Therefore, it will be appreciated that the scope ofthe present invention fully encompasses other embodiments which maybecome obvious to those skilled in the art, and that the scope of thepresent invention is accordingly to be limited by nothing other than theappended claims, in which reference to an element in the singular is notintended to mean “one and only one” unless explicitly so stated, butrather “one or more.” All structural, chemical, and functionalequivalents to the elements of the above-described preferred embodimentthat are known to those of ordinary skill in the art are expresslyincorporated herein by reference and are intended to be encompassed bythe present claims. Moreover, it is not necessary for a device or methodto address each and every problem sought to be solved by the presentinvention, for it to be encompassed by the present claims. Furthermore,no element, component, or method step in the present disclosure isintended to be dedicated to the public regardless of whether theelement, component, or method step is explicitly recited in the claims.No claim element herein is to be construed under the provisions of 35U.S.C. 112, sixth paragraph, unless the element is expressly recitedusing the phrase “means for.”

We claim:
 1. An adsorption material, comprising: a porous metal-organicframework; and a plurality of ligands within the pores of themetal-organic framework, each ligand having at least one basic nitrogengroup; wherein the basic nitrogen groups are configured to selectivelyadsorb CO₂ from a stream of mixed gases at pressures below 3 bar and CO₂partial pressures between 1 and 1000 mbar.
 2. A method of separating amixture stream comprising CO₂ and N₂, the method comprising: contactingthe mixture stream comprising CO₂ and N₂ with a material comprising ametal-organic framework, and a ligand with a basic nitrogen group;wherein the material has an isosteric heat of CO₂ adsorption of greaterthan −60 kJ/mol at zero coverage using a dual-site Langmuir model;obtaining a stream richer in CO₂ as compared to the mixture stream; andobtaining a stream richer in N₂ as compared to the mixture stream.
 3. Amethod of separating a mixture stream comprising CO₂ and othercombustion gases, the method comprising: contacting the mixture streamcomprising CO₂ and N₂ with a material comprising a metal-organicframework and a plurality of ligands that have at least one basicnitrogen group; obtaining a stream richer in CO₂ as compared to themixture stream; and obtaining a stream richer in N₂ as compared to themixture stream.